X-ray imaging system has found wide applications in various areas, including medicine, material analysis, and security. X-ray detector is the key component of the x-ray imaging system.
FIG. 1 is a diagram illustrating the structure and principle of a conventional photodetector including a photodetector substrate 111 and a readout circuit 6. The photodetector substrate 111 in FIG. 1 includes a base substrate 11, an array of thin film transistors 5 on the base substrate 11, a photon absorbing layer 1 on the array of thin film transistors 5, and multiple first electrodes 2 and second electrodes 3 between the array of thin film transistors 5 and the photon absorbing layer 1. The gate electrode and the drain electrode of each thin film transistor 5 are connected to the readout circuit 6. The drain electrode 6 is connected to ground through a storage capacitor 4, the other terminal of which is connected to the first electrode 2. The second electrode 3 is at positive potential.
When a photon having the right wavelength is absorbed by the photon absorbing layer 1, an electron-hole pair is produced. The holes migrate to the first electrode 2 under the electric field between the first electrode 2 and the second electrode 3, and are stored in the storage capacitor 4. The readout circuit 6 turns on the thin film transistors 5 sequentially, and read out the charge stored in the storage capacitor 4. The charge stored in each storage capacitor 4 correlates with the amount of photons absorbed in the corresponding photo absorbing layer 1.
Electron-hole pairs can also be randomly produced by temperature variation in the photo absorbing, layer 1, which results in dark current noise. The dark current noise may affect detection accuracy of the photodetector.